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Paris, 27 February 2012

Drilling in the Lesser Antilles will improve assessment of risks related to volcano instability

From March 3 to April 17 2012, an international team jointly led by Anne Le Friant, a CNRS researcher at the Institut de Physique du Globe de Paris (CNRS / Université Paris Diderot / IPGP) and by Ozamu Ishizuka of the Geological Survey of Japan will embark on an oceanographic campaign in the Lesser Antilles. This initiative aims to better evaluate the risks associated with volcano flank instability, which can cause tsunamis. To achieve this, the scientists will drill around ten boreholes, which will be used to reconstruct the eruption history of the most active regions in the Antilles over the last million years. For the first time ever, core sampling will be carried out on a seabed covered with volcanic debris avalanche deposits.

The Lesser Antilles: a volcanic arc with multiple risksThe Lesser Antilles arc(1), which is directly related to the plate boundary between the Caribbean and North American plates, is made up of numerous volcanic edifices of which at least twelve have been active in the past 10,000 years. Montagne Pelée, in Martinique, and La Soufrière, in Guadeloupe, are among these volcanoes. Although magma compositions and eruption styles vary all along the arc, eruptions are usually explosive. Major landslides (also known as 'collapses'(2)) can occur on the flanks of volcanoes, causing potential tidal waves when the debris avalanche enters the sea. Scientists have previously shown that volcanoes in the Lesser Antilles have suffered at least 52 flank collapses, 15 of which took place in the last 12,000 years. Around Montserrat, no less than 75% of the products emitted by the ongoing eruption of the volcano have flowed into the sea.

Drilling offshore to reconstruct eruption history and understand the risksDuring the new IODP(3) campaign, scheduled from March 3 to April 17 2012, the team plans to drill ten boreholes (130 – 500 meters deep), at strategic spots around three sites that are representative of the main volcanic processes in the Lesser Antilles arc: Montserrat, Martinique and Dominica. By analyzing marine core samples collected in 2002 off the Lesser Antilles, the scientists discovered a larger number of eruptions than had been inferred from surveys carried out on land only (where eruption deposits are sometimes hidden or eroded). In addition to sediments and layers of volcanic ash that can be used to trace the volcanoes' history, the drilling will for the first time make it possible to collect samples of debris avalanche deposits, in an area where the frequency of collapses appears greater than elsewhere.

The campaign's objective is to achieve the most detailed reconstruction possible of the eruption history of the Lesser Antilles volcanoes by documenting the volcanic construction and destruction cycles. In particular, the scientists will attempt to better define the nature of volcanism during the first stages of volcanic edifice construction (chemical composition, production rate, explosiveness, role of construction compared with destruction), as well as the processes characterizing eruption activity and its migration along the Lesser Antilles arc. The aim is also to better understand the processes at work in debris avalanches and sediment dispersion in the ocean environment. This information will help to improve assessment of the hazards related to volcanic activity in the region.

Location of planned drilling sites. Core drilling (130-500 meters depth) will be carried out off Martinique, Dominica and Montserrat, in both debris avalanches and sediments and tephra (layers of volcanic ash).

Changes in submarine topography off the coast of Montserrat during the ongoing eruption of the Soufrière Hills volcano:
bathymetric data (depth of the seafloor) from successive surveys carried out in 1999, 2002, and 2005 during various oceanographic campaigns, used to monitor the deposition of material that flowed into the sea during the eruption. The color scale shows the thickness of the deposits laid down from 1999 to 2002 (a), and from 2002 to 2005 (b).

Notes:

1 - As opposed to the Greater Antilles, the Lesser Antilles is an archipelago made up of the southernmost islands in the Antilles, from the Anegada Trench to the north of the South American subcontinent. Guadeloupe and Martinique are part of the Lesser Antilles.
2 - The collapse is a result of various processes: injection of magma that fails to reach the surface, weakening of the flank by hydrothermal alteration, and fracture or overload caused by the edifice.
3 - The Integrated Ocean Drilling Program (IODP) is an ambitious project in which France, along with 17 other countries, plays a major part via the European ECORD consortium.